A function that takes the limits as input and returns breaks
as output

limits

A character vector that defines possible values of the scale
and their order.

drop

Should unused factor levels be omitted from the scale?
The default, TRUE, uses the levels that appear in the data;
FALSE uses all the levels in the factor.

na.translate

Unlike continuous scales, discrete scales can easily show
missing values, and do so by default. If you want to remove missing values
from a discrete scale, specify na.translate = FALSE.

na.value

If na.translate = TRUE, what value aesthetic
value should missing be displayed as? Does not apply to position scales
where NA is always placed at the far right.

scale_name

The name of the scale

name

The name of the scale. Used as the axis or legend title. If
waiver(), the default, the name of the scale is taken from the first
mapping used for that aesthetic. If NULL, the legend title will be
omitted.

A function that takes the breaks as input and returns labels
as output

guide

A function used to create a guide or its name. See
guides() for more info.

super

The super class to use for the constructed scale

values

a set of aesthetic values to map data values to. If this
is a named vector, then the values will be matched based on the names.
If unnamed, values will be matched in order (usually alphabetical) with
the limits of the scale. Any data values that don't match will be
given na.value.

aesthetics

Character string or vector of character strings listing the
name(s) of the aesthetic(s) that this scale works with. This can be useful, for
example, to apply colour settings to the colour and fill aesthetics at the
same time, via aesthetics = c("colour", "fill").

Details

The functions scale_colour_manual(), scale_fill_manual(), scale_size_manual(),
etc. work on the aesthetics specified in the scale name: colour, fill, size,
etc. However, the functions scale_colour_manual() and scale_fill_manual() also
have an optional aesthetics argument that can be used to define both colour and
fill aesthetic mappings via a single function call (see examples). The function
scale_discrete_manual() is a generic scale that can work with any aesthetic or set
of aesthetics provided via the aesthetics argument.